1. Field of the Invention
The present invention relates to electronic imaging systems and, more specifically, to correcting projected or displayed images.
2. Background Information
Currently, there are a wide-variety of digital image projectors commercially available. Most digital projectors include a video decoder and a light engine. The video decoder converts video data received by the projector, e.g., from the display connection of a personal computer (PC), into pixel and color data. The pixel and color data is then supplied to the light engine, which converts that data into the actual projected image. The light engine includes a lamp, optics and logic for manipulating the light in order to generate the pixels and color.
There are three different types of technologies utilized by the light engines of today's projectors: Liquid Crystal Display (LCD), Digital Light Processing (DLP) and Liquid Crystal on Silicon (LCOS). An LCD light engine breaks down the light from a lamp into red, green and blue components. Each color is then polarized and sent to one or more liquid crystal panels that turn the pixels on and off, depending on the image being produced. An optic system then recombines the three color signals and projects the final image to a screen or other surface.
DLP technology was developed by Texas Instruments, Inc. of Dallas, Tex. A DLP light engine directs white light from a lamp onto a color wheel producing red, green, blue and white light. The colored light is then passed to a Digital Micromirror Device (DMD), which is an array of miniature mirrors capable of tilting back-and-forth on a hinge. Each mirror corresponds to a pixel of the projected image. To turn a pixel on, the respective mirror reflects the light into the engine's optics. To turn a pixel off, the mirror reflects the light away from the optics.
A LCOS light engine combines LCD panels with a low cost silicon backplane to obtain resolutions that are typically higher than LCD or DLP projectors. The LCOS light engine has a lamp whose light is sent to a prism, polarized, and then sent to a LCOS chip.
The LCOS chip reflects the light into the engine's optics where the color signals are recombined to form the projected image.
The quality of a projected image is a function of several characteristics, including brightness, also referred to as luminance. Due to the design of the optics within the light engines and/or the lamps themselves, most projectors do not project at a constant luminance level across the entire screen. FIG. 1 is a highly schematic illustration of the luminance levels measured in foot-lamberts of a displayed image 100 that was generated by a projector set to display all pixels at a constant luminance or brightness level. The displayed image 100 has a generally rectangular shape comprising a top edge 102, a right side edge 104, a bottom edge 106 and a left side edge 108. Rather than having a constant luminance throughout, the image 100 has a brightest region 110, which is at approximately 34.0 foot-lamberts, and several regions of decreasing luminance, terminating at a darkest region 112 at the image's upper left corner, which is at approximately 17.8 foot-lamberts. This non-uniformity in luminance detracts from the displayed image.
The luminance non-uniformity of a projector can become more pronounced under certain conditions. For example, when a “composite” image is created by multiple projectors whose individual images are tiled together, e.g., in a 4 by 5 pattern, to form the composite image, the non-uniformity in luminance is often much more apparent. Luminance non-uniformities can also be created (or existing non-uniformities made more pronounced) when the projector is set up at an angle to the display screen or surface. That portion of the displayed image that is closer to the projector will typically be brighter, while those portions located further away will be dimmer.
In addition, some projectors cause unnatural artifacts to appear in projected images. For example, if an input image has a series of regions that vary slowly in luminance, false contours or “bands” that are not present in the input image may nonetheless appear in the projected image. FIG. 2 illustrates an input image 200 whose luminance varies smoothly and gradually from a darkest region 202 near a lower left corner 204 to a lightest region 206 near an upper left corner 208. FIG. 3 illustrates a projected image 300 generated from input image 200 (FIG. 2). Rather than show an image whose luminance varies smoothly, projected image 300 has a plurality of discrete bands or false contours, including a darkest band 302, a lightest band 304, and a plurality of intermediary bands 306a-i. The appearance of such false contours, which is also referred to as “banding”, occurs when the projector lacks sufficient luminance levels to adequately represent the input image 200. The appearance of false contours can substantially detract from the quality of the projected image.
Accordingly, a need exists for a projector whose displayed image(s) is uniform in luminosity, and eliminates or at least reduces the occurrence of false contours and other spurious artifacts.